What can I do to blend the transition between 2 materials in specific parts of a mesh in Cycles?
Try using a single material with both your material setups connected to a mix shader node, and then use:
a. a texture for your model, made with the texture painter after you've unwrapped your model
b. use the vertex colours, set the brush to black or white and paint where you want your materials to be.
and connect either of those to the factor of your mix shader